Sail making is the art of compromise. The sail maker is concerned with the stretch characteristics of the sail material. Sails are typically constructed by shaping and joining together panels of material, so as to present a relatively controllable aerofoil when tensioned and subject to different wind strengths.
Until the discovery of synthetic fabrics, sails were traditionally produced from various types of canvas. Today, virtually all fore and aft sails (other than spinnakers) are made from polyester fabrics, in particular polyethylene tetraphthalate (trade names: Dacron, Terylene). With sails made from this polyester woven fabric, it is generally convenient to provide maximum directional strength along the weft of the fabric. Even though both the warp and weft stability of the fabric may be controlled, the fabric will exhibit bias stretch, i.e. along the diagonal of the matrix defined by the generally orthogonal warp and weft yarns.
Designers have concentrated on the stability of the leech in both headsails and mainsails, as the leech is generally unsupported, unlike the luff of a headsail which is tensioned by a fore stay, or the luff of the mainsail which is held by the mast.
Until the turn of the century triangular sails were scotch-cut, i.e. with the sail panels lying parallel with the leech. This meant that the warp was was parallel to the leech, and the panels met the luff and the foot on the bias. Ratsey made a significant improvement to said design when he discovered that weft stretch was more predictable than that of the warp and in his design he laid the panels at 90 degrees to the leech, thus lining up the stable weft yarns between the head and clew of the sail to stabilize the leech. Indeed, Ratsey, in his 1894 patent disclosed the concept of the mitre cut in which the panels are arranged so that the weft threads are parallel to the leech, and a separate set of panels are arranged with their weft threads parallel with the foot, the two sets of panels meeting along a mitre line.
The mitre cut and the more recent cross cut (in which all the panels have the weft parallel to the leech) result in bias stretch in the luff area of the sail, i.e. the area between the head and tack bounded by the luff (edge) and the draft (the point of maximum camber when under sail).